Corrosion on the surface, the etching, pitting, eating away of a material such as the surface of plaster. Etching can be caused by chemical action on the surface side, mechanical action from within the plaster mix, or from the shell side. An example of severe etching occurs when a plaster pool is ACID WASHED. ACID is used to clean the surface by eating away the surface that is stained, leaving the underneath surface exposed, porous (and hence, susceptible to stains), and (usually) rough. Severe CHEMICAL IMBALANCE can also cause etching. SEE ALSO "Ideal Chemical Balances "